This invention relates generally to electronic amplifiers, and more particularly to variable gain amplifiers (VGAs).
A certain group of variable gain amplifiers (VGA) convert a differential input voltage to a differential output voltage in response to the magnitude of a control voltage. VGAs are useful in a variety of analog electronic applications, many of which require a high degree of logarithmic transfer function linearity, i.e. where the control voltage varies linearly on a linear (x) axis, the amplitude varies linearly on a logarithmic (y) axis. This is known as a log-linear or linear-in-dB transfer function. As discussed below, prior art VGAs tend not to provide good linear-in-dB behavior (even to a "first order") over a wide dynamic range.
In FIG. 1, a VGA 10 of the prior art includes four bipolar NPN transistors 12, 14, 16 and 18. Because it has four transistors, VGAs such as VGA 10 are often referred to as "quad" VGAs. The collectors of transistors 12 and 18 are coupled directly to a power source (i.e. V.sub.cc), and the collectors of transistors 14 and 16 are coupled to V.sub.cc by resistors 20 and 22, respectively. The output voltage V.sub.o is developed across the collectors of transistors 14 and 16.
The bases of transistors 12-14 are controlled by an internally developed differential control voltage V.sub.c. That is, a line 24 connected to a +V.sub.c node is connected to the bases of transistors 14 and 16, while the -V.sub.c node is connected to the bases of transistors 12 and 18 by a line 26. The differential control voltage V.sub.c is developed by circuitry 28 connected to an external pin 30 of an integrated circuit device. An external control voltage V.sub.CEXT is applied to the pin 30 to control the gain of the VGA.
The circuitry 28 can range from the very simple to the relatively complex depending upon the amount and type of signal processing that is desired. In its simplest form, the circuitry 28 can include coupling line 24 to a fixed voltage, and a connection between the pin 30 and line 26. In a more complex design, the circuitry 28 includes circuitry for temperature compensation and/or circuitry for adjusting the slope of the transfer curve ("transfer function") of the VGA 10.
A differential input voltage V.sub.i is input to a voltage-to-current converter (g.sub.m) to produce the currents i.sub.1.sup.+ on a line 32 and i.sub.1.sup.- on a line 34. The line 32 is coupled to the emitters of transistors 12 and 14 while the line 34 is coupled to the emitters of transistors 16 and 18.
The operation of the VGA 10 of the prior art will be described with reference to the transfer function of FIG. 2. The equation for the transfer function is given below in Equation 1. ##EQU1## where V.sub.T =kT/q.
In operation, a control signal V.sub.CEXT is applied to pin 30 to determine the degree of attenuation of the input signal. On the y axis of the graph of FIG. 2 is the current ratio i.sub.o /i.sub.i, which is measured in decibels (dB), and on the x axis of the graph is the control voltage V.sub.c, as measured in volts. As can be seen, the transfer function 36 is essentially linear-in-dB in a region 38 where V.sub.c is negative. Above approximately V.sub.c =0, i.e. in the positive region for V.sub.c, a non-linear or "compressed" region 40 asymptotically approaches 0 dB.
For applications where a high degree of transfer function linearity is required, the prior art had little option but to operate the VGA 10 only within the linear region 38. However, this reduces the dynamic range of the VGA 10. It would therefore be desirable to have a VGA design which extends the dynamic range of the VGA so that it can operate linearly in regions where